A method performed in a medical navigation system includes driving a transmitter at a first frequency and a second frequency to generate first and second electromagnetic fields, wherein the first and second frequencies are sufficiently low such that the first and second electromagnetic fields are frequency independent; receiving first and second distorted fields corresponding to the first and second electromagnetic fields, respectively, with each of at least two electromagnetic (EM) sensors attached to a surgical device; generating first and second signals in response to receiving the first and second distorted fields, respectively, using each of the at least two EM sensors; and determining a distortion in the first and second signals based at least on a distance between the at least two EM sensors and a difference between the first and second signals generated by each of the at least two EM sensors.